Let me let you into Westminster’s worst kept secret. There is no chance – absolutely no chance – of Jeremy Corbyn leading Labour into the 2020 General Election. Corbyn is 66 now; at the time of the next election, he’ll be a few weeks short of seventy-one. The demands of modern political life make leadership no place for a septuagenarian.

But even if Corbyn were younger, there’d be no chance then either. This is someone who’s spent his entire political life on the backbenches, immersed in the politics of protest. He’ll never have even dreamt of making it to the Labour front bench, let alone the leadership; more to the point, he won’t have wanted to either. His performance (sic) so far as leader is an ample illustration why: Corbyn knows he’s been wildly over-promoted; that he isn’t up to the task of a job which requires authority and control over his party and its message. This is The Peter Principle writ large.

The Tories know it: that’s why they’ve already started insinuating that not Corbyn, but Labour en masse, are a “threat to security”. Dan Hodges knows it; Nick Cohen has called for it; Stephen Bush, as he hinted on Thursday, knows it too. Absurd though it might sound only a week into Corbyn’s leadership, the question for Labour is (1) How to replace him; (2) When; (3) With whom?

Two theories are doing the rounds. One involves Tom Watson (as she was the first to suggest it, even while the leadership contest was still ongoing, let’s call it the Mensch Thesis); the other, Alan Johnson. Let’s call that the Pedley Thesis, and take the second one first. On Thursday, Keiran Pedley set out his view that the genial Johnson, longtime Tony Blair loyalist, former General Secretary of the Union of Communication Workers, and who can now often be found making merry on Andrew Neil’s comfy sofa, could be the man Labour coalesce around midway through the Parliament, much as the Tories did with Michael Howard in 2003.

In my view, since Denis Healey was fatefully rejected in 1980, Labour have only produced three individuals who could have won a General Election in most circumstances: Blair of course, David Miliband… and that man Johnson. A major part of why Blair hung on so long after the Iraq debacle was his desperation that someone emerged who could stop Gordon Brown making a Horlicks of all his and New Labour’s good work. With his innate, preternatural political instincts as sharp as ever, Blair knew Brown wasn’t remotely successful Prime Ministerial material; but one man in his Cabinet was.

When, following an internal coup against him fronted by Watson in 2006, Blair announced preparations for his departure the following year, who was the friend by his side? Johnson. This wasn’t a coincidence. Over the nine-and-a-half months which remained, given Miliband was still too wet behind the ears, Blair must’ve hoped that Johnson could rise in prominence enough to credibly challenge Brown for the succession – but there was a problem. An insurmountable one. Johnson didn’t want the job. He never has.

British political history is full of counter-factual talking points. If Margaret Thatcher hadn’t been forced out by her own party, and instead allowed to lead it to defeat in 1992, the fratricidal nature of her exit wouldn’t have poisoned the Tories for so long afterwards. If Labour had won the 1992 election, but then faced Black Wednesday just five months later, goodness knows what existential damage this would’ve wrought. If John Smith had lived, Labour would never have moved so far rightwards and alienated (until last weekend, that is), so much of its core support. If Brown had called a snap election in 2007, David Cameron would almost certainly have been defeated, ousted by his party in favour of a right-winger, and Labour might well still be in power even now.

That Brown didn’t – he bottled it because of Cameron’s inheritance tax pledge – remains the most significant political watershed of this generation. His paralysis in the face of a glaring opportunity confirmed Blair’s worst fears and left Labour on the rack. They’ve never recovered since.

But in Summer 2009, as Brown reeled from catastrophic local and European results, there was another chance. Various ministers resigned; The Guardian called for the Prime Minister to do the honourable thing. With Labour hopelessly placed in the polls, perhaps recognising what a hospital pass the premiership seemed to represent, now – pace Michael Portillo 1995 – it was Miliband’s turn to dither. And again, Johnson clearly wasn’t interested.

Between then and the 2010 election, the horrendously unpopular Brown turned an insurmountable 20-point deficit into, with the aid of the Clegg bounce, a hung Parliament. Imagine how much better the outcome would’ve been for Labour had either Miliband or Johnson grasped the nettle: as a minimum, they’d have surely been the largest party. No Cameron and Osborne; no Liberal Democrat wipe-out this year; no Tory-led slash and burn, but Labour-led trim and singe instead.

Even then, after the wrong brother won the Labour leadership, there were further opportunities to oust the poorly performing Ed during the last Parliament. With David having exiled himself in New York, once more, the only man Labour could realistically have united around was Johnson; once more, loyalty to his party and his own very real humility trumped any ambitions which, to reiterate, he’s never held anyway.

At any point between 2007 and last weekend, Alan Johnson could’ve been the Howard-esque unifying figure which Labour will desperately require post-Corbyn. Had he succeeded Blair, Johnson – with broad appeal across the country and throughout his party, as well as a compelling backstory – could’ve been Labour’s John Major: a vastly more successful one too. But last weekend – and here’s where the Pedley Thesis collapses – the world tilted entirely on its axis.

By opening the leadership contest up, Ed Miliband’s unwitting legacy to the Labour Party was, in effect, its death; and the immediate, Phoenix-like emergence of a new, grassroots-based movement of the left. A long overdue fusion of traditionalism and radical populism. Labour was never going to be miraculously immune from the anti-austerity, anti-neoliberal forces which have propelled the rise of Syriza, Podemos, the SNP or Bernie Sanders; nor the sterilisis which has plagued its social democratic sister parties across Europe. As Britain’s increasingly absurd electoral system strangles new parties at birth, this fusion was the only answer for the left. A personal view is its long term outcome – 10 or 15 years from now, with or without Scotland – will be the implementation of proportional representation.

What does this mean? Put simply, the Blairite wing of the party is finished, for good. That’s why we’re hearing rumours of disgruntled Blairites crossing the floor: they know the game is up. Although he’d furiously protest to the contrary, I suspect this is part of what’s put Cohen’s nose so out of joint; it certainly has with John “who cares about the grassroots?” McTernan. But Johnson, however popular and likeable he remains, is himself a Blairite; and would certainly be seen as such by an infuriated, much expanded membership in the event of Corbyn’s departure.

On Wednesday, Hodges wondered whether “there’s a choice for Labour other than simply embracing Corbynism or returning to Blairism?” The always perceptive Telegraph columnist has already sensed which way the wind’s blowing. What’s happened isn’t just about Corbyn at all; the changes in Labour’s make-up are seismic and irreversible. Centrifugal forces cannot be resisted: there can never be a return to the top-down triangulation of the past.

Where I differ from Stephen Bush, though, is I’d argue that Labour’s soft left were in full-blown retreat after Blair’s accession in 1994; all but vanished altogether; then started making small, baby steps forward during Miliband’s ill-starred leadership. Now, not so much the hard left (who, other than John McDonnell, is of the hard left and among Corbyn’s more senior Shadow Cabinet members?) as the soft left are triumphant.

Not for nothing did Neal Lawson state Labour were as good as dead only to publicly declare for Corbyn, to say nothing of Anthony Barnett and Jeremy Gilbert chiming in with their support: along with, in the latter’s case, his vehement disdain for Blairism. The often mischaracterised Owen Jones, Zoe Williams, Paul Mason and Aditya Chakrabortty are all in the vanguard of the modern soft left; all are hugely supportive of Corbyn, and especially energised by the long term opportunity his victory represents.

Longer term, there’s nothing to stop, say, Stella Creasy, the epitome of the soft left, becoming Labour leader after 2020. She recognises how greatly the party needs to change, always emphasises the need for a grassroots movement, is – hallelujah – in favour of electoral reform, and as her vote in the Deputy Leadership contest proved, is popular among the membership: just not as much as one individual. The man at the centre of the Mensch Thesis, Tom Watson.

Few individuals have brought down one Labour leader, let alone two. Even fewer have gone on to inherit the crown themselves. But equally: few have held as much power within the Labour Party as Watson now does. Massively popular among the membership thanks to his brilliant campaigning on the Westminster paedophile ring, questioning of the Murdochs, and ousting of Blair, Labour’s fixer commands respect across the political spectrum; and to judge by his comments during the Deputy Leadership contest, has at least modified his machine-like ways of the past.

He is also a personal friend of Louise Mensch, Twitter Tory attack dog-in-chief. Mensch is frequently a hate figure for the left (even, on occasion, a joke figure); but this is to seriously underestimate her very real qualities of political cunning and nous. She’s an increasingly shrewd observer of the Westminster scene.

On August 9, Mensch explained how Watson could very quickly replace Corbyn as leader: even as early as Christmas. On Tuesday, she went much further: noting that trade union leaders were already subtly distancing themselves from Corbyn after his disastrous first few days in the job; and reminding us that Watson, not the new leader, is the man who actually has most of them in his pocket.

On Thursday, Bush characterised Watson as on the party’s ‘soft right’. I think that’s a mistake. Just as Creasy is sometimes strangely referred to as a Blairite (she’s not; she’s a non-dogmatic, labels-rejecting, all-inclusive Labourite), Watson is sometimes oddly seen as on the right of the party. In fact, he’s a reformed Brownite, parked right in the centre not only of the Parliamentary Labour Party, but the membership too. The unions’ man; the PLP’s man; the membership’s man. He could not be more strongly positioned.

And that he’s arguably the only figure capable of bridging the yawning chasm separating much of the bewildered, blindsided PLP from the now massively leftist membership is of critical importance. Whenever Corbyn goes (even if it’s genuinely of his own volition, or follows electoral meltdown next year or in 2017), many members will be furious and out for blood. Meanwhile, David Cameron’s reforms to trade union funding – a naked attempt to destroy the Labour Party, as Hodges has set out – mean Labour literally cannot afford to lose those members. It needs them, and many more of them. The only way it can survive, change and ultimately prosper is by broadening its reach across the British left, and eventually uniting it. Corbyn’s victory was merely the first step in that long, hugely overdue process.

How can Labour avoid civil war among the membership and make itself electorally credible again? Just as the Tories did with Howard: by crowning Watson a year or (more likely) two from now, without a contest. Watson already has a mandate; as Corbyn’s Deputy, it’s natural he should succeed. Then, just as Howard didn’t change much of Iain Duncan Smith’s message – he merely tweaked it at the edges, while communicating it infinitely more effectively – Watson wouldn’t need to change all that much of Corbyn’s message either. Somewhere to the right of Corbyn and the left of Miliband – a very distinctive message, voiced in a credible way by a competent, strong leader – is what would result.

Could a Watson-led Labour win in 2020? No, especially given the boundary changes and gerrymandering which the Tories will push through. But he can emulate Howard in overseeing the start of the recovery, going down to reasonable defeat, and handing over to a younger, longer term figure afterwards: either Creasy or Dan Jarvis, in all likelihood.

Of course, a great deal can change in two years in politics. Britain might well be mired in recession by 2017; new figures may have emerged in the party. But right now, it is incredibly difficult to see who other than Watson can replace Corbyn and keep Labour together. Everything points to him; and Blairites most certainly need not apply. Louise Mensch 1, Keiran Pedley 0.